1. Field of the Invention
This invention relates to a condenser microphone, more particular to a condenser microphone including a spacer unit that is formed with a plurality of air passages and to a method for making the same.
2. Description of the Related Art
FIG. 1 illustrates a conventional condenser microphone that is formed by micro-machining technology and that includes a variable gap condenser mounted in a condenser housing 300. The variable gap condenser includes a silicon substrate 30 mounted on a bottom wall of the condenser housing 300, an insulating substrate 31 formed on the silicon substrate 30 and formed with a buffering recess 311, a conductive back plate 32 formed on the insulating substrate 31 and defining a first electrode of the condenser microphone, a spacer 33 formed on the back plate 32, a compliant layer 34 formed on the spacer 33 and cooperating with the spacer 33 and the back plate 32 to define a variable gap 36 thereamong that is surrounded by the spacer 33, and a conductive film 35 formed on the compliant layer 34 and defining a second electrode of the condenser microphone. A field effect transistor 200 is formed on the insulating substrate 31, and is connected electrically to the conductive film 35 so that a change in capacitance of the variable gap condenser due to vibration of the compliant layer 34 by an incoming sound wave can be converted into a corresponding electrical signal.
A plurality of through-holes 321 are formed in the back plate 32 and a top wall of the insulating substrate 31 so as to permit spatial communication between the variable gap 36 and the buffering recess 311 in the insulating substrate 31 and so as to reduce air stream resistance in the variable gap 36 during microphone operation.
The conventional condenser microphone is disadvantageous in that formation of the buffering recess 311, which normally has a depth ranging from 300 to 500 microns, in the insulating substrate 31 involves bulk etching process, which is time-consuming. Moreover, formation of the aforesaid through-holes 321 further increases the production time.